Visual contents, whether fixed or moving images, are generally creations that benefit from guarantees of exclusivity related to the copyright. In general, they can only be reproduced within a strictly defined framework that enables the authors and their entitled parties to be remunerated.
In order to ensure that these legal rules are correctly observed, many systems have been developed to prevent illegal copies or to deteriorate their quality sufficiently to render them unusable. Within this framework, a method and a device aiming to fight against the copying of images by scene when they are displayed, for example by a camcorder in a cinema venue, were described in the patent application WO 05/027529. The method proposed in this patent application plans to modulate the colour (or chrominance) of the pixels of a pattern to display at a high frequency that makes the pattern invisible to the human eye but generates artifacts on the sequence filmed by the camcorder. Such a solution requires doubling at least the display frequency of the images so that the modulation is not visible. In practice, this procedure, called anti-camcorder method in the rest of the description, consists for example of decomposing each source image into two complementary images and doubling the display frequency such that the display of these two images is perceived as a single image (corresponding to the source image) by the human eye. Complementary image is understood to mean an image in which the colour of some pixels (representing an anti-copy pattern or message) is modified, the resulting colour of these pixels, after integration of the two complementary images, is the colour of the pixel of the initial source image.
Such an anti-camcorder method requires that the number of images displayed is at least twice the number of initial source images. This operation can only be carried out at the level of the projection system itself to be compliant with DCI (Digital Cinema Initiatives) recommendations for digital cinema.
Other anti-camcorder methods based on metamerism and using at least one fourth primary colour to display the images are also known. Such a method is revealed in the patent application WO 2004/040899. The use of such a method requires converting the source images defined in 3 primary colours into images defined into a larger number of primary colours, for example 4 primary colours. As with the anti-camcorder method described previously, this processing of the source images is currently carried out in the projection system in real time (at the time of display).
This video processing is relatively complex and can give rise to a significant increase in the cost of the projection system.